Ionic Cross‐Linking and Its Influence on the Properties of Self‐Doped Conjugated Polyelectrolytes

ABSTRACT

Self‐doped conjugated polyelectrolytes (CPEs) offer a unique combination of π‐conjugated backbones and ionic side chains, enabling solution‐processability, mixed ionic‐electronic conductivity, and applications in aqueous environments. However, their high water solubility hinders long‐term stability, necessitating effective cross‐linking strategies. Here, we systematically investigate ionic cross‐linking of the benchmark p‐type self‐doped poly[2,6‐(4,4‐bis‐potassium butanylsulfonate‐4H‐cyclopenta‐[2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], CPE‐K, using a post‐deposition approach with divalent cations (Mg ^{2 +} , Ca ^{2 +} , Sr ^{2 +} , Ba ^{2 +} ). Thin films immersed in aqueous salt solutions exhibit complete ion exchange, as confirmed by XPS. AFM and liquid‐mode AFM reveal that swelling suppression and morphological stability depend strongly on the cation identity and concentration, with Ba ^{2 +} forming the most rigid cross‐linking network. GIWAXS shows that ionic cross‐linking expands lamellar spacing while preserving π – π stacking. Electronic conductivity measurements demonstrate that Ba ^{2 +} ‐treated films maintain high conductivity in the swollen state due to improved network integrity. Impedance spectroscopy further shows decreasing ionic conductivity with increasing cross‐linking density. These findings establish structure–property relationships for ionic cross‐linking in CPEs and provide design insights into processing protocols to form robust CPE films caste from water.